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I don’t think .osm files are a good choice as a data source for drawing maps on a mobile device. The process to render a map from raw OSM data is quite complicated, I would consider pre-processing it to a simpler format. (Even though if JavaScript can do it…)

I am not aware of any iPhone-ready libraries/solutions, but if you want to explore, see the Rendering page on the OpenStreetMap wiki, which lists many examples of the available rendering software.